The deputy children's commissioner said children's access to pornography on their mobiles was affecting the thresholds of what they considered normal sexual behaviour. Photograph: Janine Wiedel Photo library/Alamy

Children are being encouraged to take part in sexual activity by exposure to hardcore pornography on their mobile phones, the deputy children's commissioner sayssaid yesterday.

Sue Berelowitz told the home affairs select committee that social networking sites and the use of pornography was one of the key areas she was examining in an investigation of group and gang child sexual exploitation. Her inquiry had already revealed that such exploitation was taking place across the country in urban, rural and metropolitan areas. "It is violent, it is sadistic, it is very, very ugly," she told MPs on Tuesday.

She said the issue of how social networking, BBM messaging and pornography was being used as part of the exploitation of children and young people – often by teenagers not much older than themselves – was of serious concern.

In one case, boys in a gang "were using BBM messages to summon other girls to them to rape and this went on for several hours," Berelowitz said. "There are parts of London where children from the age of 11 expect to have to carry out oral sex on a lineup of boys."

She said she was concerned about the viewing of pornography by young people, which contributed to the problem of child sexual exploitation.

"They are watching it and then they are enacting it. Parents think they know what their child is watching … the reality is children can get anything they like on their mobile phones and they are. This is affecting children's thresholds of what they think is normal behaviour.

"Social networking sites can be a source of real problems in this area. They [the perpetrators] are sometimes filming their victims, girls are making themselves vulnerable by filming themselves …"

Berelowitz was giving evidence to the committee which is investigating the issue of street grooming and child sexual exploitation after nine British Asian men in Rochdale were convicted for grooming and abusing vulnerable young girls.

The case – along with several others in the north of England over the last three years — has provided some evidence that street grooming of young girls, often from children's homes, is disproportionately being carried out by Asian men who target white victims. The five victims – the youngest was 13 when the abuse began – were plied with food, alcohol, drugs and gifts so they could be passed around a group of men for sex.

The defendants were jailed for a total of 77 years last month, with the ringleader, a 59-year-old man from Oldham who cannot be identified for legal reasons, receiving a 19-year term after being convicted of two rapes, aiding and abetting rape, sexual assault and trafficking for the purpose of sexual exploitation.

Berelowitz told the committee the issue of ethnicity was complex. She said there was a particular pattern of Asian men and white girls as revealed by the Rochdale case and others, but she said it was a pattern among many other patterns of child sexual exploitation.

"In terms of victims, while people have exclusively here talked about white victims, I am finding that girls from all ethnic groups are victims. So girls from black and ethnic minority groups are definitely victims, and they are too often missed out. They are really not being properly identified as victims.

"There are people from all backgrounds – white, Pakistani, Afghan, traveller you name it – who are seeing chidren as easy access in terms of sexual exploitation."